Greenwich Mean Time, also known as GMT, is the official time recorded at the Royal Observatory in Greenwich, England. Greenwich, England, sits on the Prime Meridian, which is zero degrees longitude, and is the official starting point from which all of the Earth's time zones are measured. GMT is called "mean" time because it measures the average, or the mean, time that it takes the Earth to rotate
. in one 24-hour period. GMT has been officially replaced by atomic time, but GMT is still in widespread use. For more information about Greenwich Mean Time, check out http://wwp.greenwichmeantime.com/.
